A theoretic framework for analyzing the morphology, physiology and mechanics of the egg cortex during cell division has been developed. The investigations under way are efforts to test this theory and substantiate it. An extensive ultrastructural analysis of the surface topography and internal cytoplasmic organization of the cortex in sea urchin and other eggs is in progress. Its goal is to define the relationships of surface area change, of microvilli on the surface, and of actin microfilaments in microvilli and elsewhere in the cortex with the processes leading to man culminating in cell cleavage. Both scanning and transmission electron microscopes are being used. The physiology and mechanics of the cortex are being examined by biophysical means in order to establish the mechanical and behavioral correlates of various morphological changes which have been observed. Furthermore, these investigations have the goal of ascertaining the validity of the postulate that the egg cortex exhibits rhythms of mechanical change which are essential to the onset of cell cleavage. These studies endeavor to elucidate the underlying mechanism of cell cleavage (cell division) in marine eggs as model cells for the understanding of cell division mechanisms in all cells.